Ptfe layers and methods of manufacturing

ABSTRACT

Single, continuous PTFE layers having lateral zones of varied characteristics are described. Some of the lateral zone embodiments may include PTFE material having little or no nodal and fibril microstructure. Methods of manufacturing PTFE layers allow for controllable permeability and porosity of the layers, in addition to other characteristics. The characteristics may vary from one lateral zone of a PTFE layer to a second lateral zone of a PTFE layer. In some embodiments, the PTFE layers may act as a barrier layer in an endovascular graft or other medical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/106,131,filed Apr. 13, 2005, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) layers have been used for the manufactureof various types of intracorporeal devices, such as vascular grafts.Such vascular grafts may be used to replace, reinforce, or bypass adiseased or injured body lumen. One conventional method of manufacturing“expanded” PTFE layers is described in U.S. Pat. No. 3,953,566 by Gore.In the methods described therein, a PTFE paste is formed by combining aPTFE resin and a lubricant. The PTFE paste may be extruded. After thelubricant is removed from the extruded paste, the PTFE article isstretched to create a porous, high strength PTFE article. The expandedPTFE layer is characterized by a porous, open microstructure that hasnodes interconnected by fibrils.

Such an expansion process increases the volume of the PTFE layer byincreasing the porosity, decreasing the density and increasing theinternodal distance between adjacent nodes in the microstructure whilenot significantly affecting the thickness of the PTFE layer. As such,the conventional methods expand the PTFE layer and impart a porosity andpermeability while only providing a negligible reduction in a thicknessof the PTFE layer. In situations where a thin PTFE layer, andspecifically, a thin PTFE layer having a low fluid permeability isneeded, conventional PTFE layers are largely unsatisfactory due to theporosity and highly permeable nature of the expanded PTFE layer.

Therefore, what have been needed are improved PTFE layers and improvedmethods for manufacturing the PTFE layers. In particular, it would bedesirable to have thin PTFE layers that have a controllable permeabilityto fluids (gases, liquids or both). It may also be desirable to havesuch thin PTFE layers that have a high degree of limpness and supplenessto allow mechanical manipulation or strain of such a PTFE layer withoutsignificant recoil or spring back.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide PTFE layers and films andmethods of manufacturing the PTFE layers and films. Embodiments of thepresent invention may include one or more layers of a fluoropolymer,such as PTFE. Embodiments of PTFE layers may include at least a portionthat does not have a significant node and fibril microstructure.

In one embodiment, a method of processing PTFE includes providing alayer of PTFE, selectively applying a stretching agent to at least onelateral zone of the layer of PTFE in a predetermined pattern andstretching the layer of PTFE. In another embodiment, a method ofprocessing PTFE includes providing a layer of PTFE having a stretchingagent content level and selectively removing stretching agent from atleast one lateral zone of the layer of PTFE in a predetermined patternand stretching the layer of PTFE. In yet another embodiment of a methodof processing PTFE, a layer of PTFE is provided. Stretching agent isapplied to at least one lateral zone of a surface of the layer in apredetermined pattern until the lateral zone is saturated withstretching agent. Next, the layer of PTFE is stretched while the lateralzone of the layer of PTFE is saturated with stretching agent.

In another embodiment, a PTFE layer includes a layer made by providing alayer of PTFE, selectively applying a stretching agent to at least onelateral zone of the layer of PTFE in a predetermined pattern andstretching the layer of PTFE. In another embodiment, a PTFE layerincludes a layer made by providing a layer of PTFE having a stretchingagent content level, selectively removing stretching agent from at leastone lateral zone of the portion of the layer of PTFE in a predeterminedpattern and stretching the layer of PTFE. In another embodiment, a PTFElayer includes a layer made by providing a layer of PTFE, applying astretching agent to at least one lateral zone of a surface of the layerin a predetermined pattern until the lateral zone is saturated withstretching agent and stretching the layer of PTFE while lateral zone ofthe layer of PTFE is saturated with stretching agent.

An embodiment of a multi-layered vascular graft includes a first tubularbody having an outer surface and an inner surface that defines an innerlumen of the vascular graft. A second tubular body having an outersurface and an inner surface is coupled to the outer surface of thefirst tubular body. At least one of the first tubular body and thesecond tubular body includes a PTFE layer having a first lateral zonewith a substantially low porosity, a low fluid permeability and nodiscernable node and fibril structure, and a second lateral zone whichis fluid-permeable and has substantial node and fibril microstructure.

In another embodiment, a tubular structure includes a layer of PTFEhaving a first lateral zone that is fluid-permeable and has asubstantial node and fibril microstructure and a second lateral zonewith a closed cell microstructure having high density regions whosegrain boundaries are directly interconnected to grain boundaries ofadjacent high density regions and having no discernable node and fibrilmicrostructure. In another embodiment, an endovascular graft includes aPTFE layer having a first lateral zone that is fluid-permeable adjacenta second lateral zone with a closed cell microstructure having highdensity regions whose grain boundaries are directly interconnected tograin boundaries of adjacent high density regions and having nodiscernable node and fibril microstructure. In yet another embodiment, aPTFE layer includes a first lateral zone with a substantially lowporosity, a low liquid permeability, no discernable node and fibrilstructure, and a high degree of limpness and suppleness to allowmechanical manipulation or strain of the PTFE layer without significantrecoil or spring back. The PTFE layer also includes a second lateralzone which is fluid-permeable and has a substantial node and fibrilmicrostructure.

These features of embodiments will become more apparent from thefollowing detailed description when taken in conjunction with theaccompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a ram extruder extruding a PTFE ribbon being taken upon a spool.

FIG. 2 illustrates a calendering process of the PTFE ribbon of FIG. 1.

FIGS. 3 and 4 illustrate a tentering process with a stretching agentbeing applied to a PTFE layer during the stretching process and withportions of the tentering machine not shown for purposes of clarity ofillustration.

FIGS. 5 and 6 illustrate a stretching process in the machine directionof the stretched PTFE layer of FIGS. 3 and 4.

FIGS. 7 and 8 illustrate a final calendering or densification processperformed on a stretched PTFE layer.

FIG. 8A illustrates a method of application of a stretching agent in apreselected pattern during a transverse stretching process in adirection that is substantially orthogonal to a machine direction by atentering machine in order to produce PTFE layers having characteristicswhich may vary across the layer in a desired pattern.

FIG. 8B illustrates as side view of the method of FIG. 8A with portionsof the tentering machine not shown for purposes of clarity ofillustration.

FIG. 8C is an enlarged view of an alternative embodiment of a portion ofthe PTFE layer of FIG. 8A containing stretching agent in a preselectedpattern, taken within the encircled portion 8C of FIG. 8A.

FIG. 8D is an enlarged view of an alternative embodiment of a portion ofthe stretched PTFE layer of FIG. 8A having a pattern of variedpermeability, taken within encircled portion 8D of FIG. 8A.

FIG. 8E is an enlarged view of an alternative embodiment of a portion ofthe PTFE layer of FIG. 8A containing stretching agent in a preselectedpattern, taken within the encircled portion 8C of FIG. 8A.

FIG. 8F is an enlarged view of an alternative embodiment of a portion ofthe stretched PTFE layer of FIG. 8A having a pattern of varied fluidpermeability, taken within encircled portion 8D of FIG. 8A.

FIG. 8G is an enlarged view of an alternative embodiment of a portion ofthe PTFE layer of FIG. 8A containing stretching agent in a preselectedpattern, taken within the encircled portion 8C of FIG. 8A.

FIG. 8H is an enlarged view of an alternative embodiment of a portion ofthe stretched PTFE layer of FIG. 8A having a pattern of varied fluidpermeability, taken within encircled portion 8D of FIG. 8A.

FIG. 8I is an enlarged view of an alternative embodiment of a portion ofthe PTFE layer of FIG. 8A containing stretching agent in a preselectedpattern, taken within the encircled portion 8C of FIG. 8A.

FIG. 8J is an enlarged view of an alternative embodiment of a portion ofthe stretched PTFE layer of FIG. 8A having a pattern of varied fluidpermeability, taken within encircled portion 8D of FIG. 8A.

FIG. 8K is an enlarged view of an alternative embodiment of a portion ofthe PTFE layer of FIG. 8A containing stretching agent in a preselectedpattern, taken within the encircled portion 8C of FIG. 8A.

FIG. 8L is an enlarged view of an alternative embodiment of a portion ofthe stretched PTFE layer of FIG. 8A having a pattern of varied fluidpermeability, taken within encircled portion 8D of FIG. 8A.

FIG. 9 is a scanning electron microscope (SEM) image of a PTFE layer ata magnification of 20,000.

FIG. 10 is a SEM image of the PTFE layer of FIG. 9 at a magnification ofFIG. 11 is a SEM image of the PTFE layer of FIG. 9 at a magnification ofFIG. 12 is a SEM image of the PTFE layer of FIG. 9 at a magnification ofFIG. 13 is a SEM image of the PTFE layer of FIG. 9 at a magnification of500.

FIG. 14 schematically illustrates a composite PTFE film that comprises aPTFE layer having low or substantially no fluid permeability and aporous PTFE layer.

FIG. 15 schematically illustrates a simplified tubular structure thatcomprises an outer layer having low or substantially no fluidpermeability and a fluid-permeable inner layer.

FIG. 16 schematically illustrates a simplified tubular structure thatcomprises a layer having low or substantially no fluid permeability anda fluid-permeable outer layer.

FIG. 17 illustrates an embodiment of an endovascular graft having anetwork of inflatable conduits.

FIGS. 18 to 20 are transverse cross sectional views of an inflatableconduit of the graft of FIG. 17.

FIG. 21 is a transverse cross sectional view of an embodiment of atubular inflatable conduit.

FIG. 22 is an elevational view that illustrates another embodiment of aninflatable endovascular graft.

FIG. 23 illustrates an embodiment of an inflatable bifurcatedendovascular

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate generally to thin PTFElayers, PTFE films, composite films having two or more PTFE layers andmethods of manufacturing the PTFE layers, films and composite films.Some particular embodiments are directed to thin PTFE layers having lowor substantially no fluid permeability with a microstructure that doesnot include significant fibril and nodal structure as is common withexpanded PTFE layers. It may also be desirable for some embodiments ofsuch thin PTFE layers that have a high degree of limpness and supplenessso to allow mechanical manipulation or strain of such a PTFE layerwithout significant recoil or spring back. Such PTFE layers may bemanufactured and used for construction of endovascular grafts or othermedical devices. For some applications, embodiments of PTFE films mayinclude one or more discrete layers of PTFE that are secured together toform a composite film. As used herein, the term “composite film”generally refers to a sheet of two or more PTFE layers that havesurfaces in contact with each other, and in some embodiments, may besecured to each other such that the PTFE layers are not easilyseparated. The individual PTFE layers used in some of the PTFE compositefilm embodiments herein may have the thinness and low fluid permeabilitycharacteristics discussed above in combination with other layers havingthe same or different properties Some PTFE layer embodiments have a lowfluid permeability while other PTFE layer embodiments have no orsubstantially no fluid permeability. A PTFE layer having a low fluidpermeability may, for some embodiments, be distinguished from thepermeability of a standard layer of expanded PTFE by comparing fluidpermeability based on Gurley test results in the form of a Gurley Numberor “Gurley Seconds”. The Gurley Seconds is determined by measuring thetime necessary for a given volume of air, typically, 25 cc, 100 cc or300 cc, to flow through a standard 1 square inch of material or filmunder a standard pressure, such as 12.4 cm column of water. Such testingmay be carried out with a Gurley Densometer, made by Gurley PrecisionInstruments, Troy, N.Y. A standard porous fluid-permeable layer ofexpanded PTFE may have a Gurley Number of less than about 15 seconds,specifically, less than about 10 seconds, where the volume of air usedis about 100 cc. In contrast, embodiments of layers of PTFE discussedherein having low fluid permeability may have a Gurley Number of greaterthan about 1500 seconds where 100 cc of air is used in the test. Anembodiment of a PTFE layer discussed herein having no or substantiallyno fluid permeability may have a Gurley Number of greater than about 12hours, or up to a Gurley Number that is essentially infinite, or toohigh to measure, indicating no measurable fluid permeability. Some PTFElayer embodiments having substantially no fluid permeability may have aGurley Number at 100 cc of air of greater than about 1×10⁶ seconds.Stretched PTFE layers processed by embodiments of methods discussedherein having no discernable node or fibril microstructure may initiallyhave substantially no fluid permeability. However, such PTFE layerembodiments may subsequently be stretched during a manufacturingprocess, such as the manufacture of an inflatable endovascular graft,during which process the PTFE layer may become more fluid-permeable andachieve a level of low permeability as discussed above.

FIGS. 1-8 illustrate processing of PTFE material to form a thin,stretched PTFE layer having low or substantially no liquid permeabilityfor particular liquids, such as water based liquids. Such an embodimentmay be useful where it is desirable to exclude water based fluids andother fluids, such as body fluids of a patient. Some PTFE layerembodiments discussed herein may also be substantially impermeable toair and other gases. As such, embodiments of the stretched PTFE layersare not “expanded” in the conventional sense as taught by Gore in U.S.Pat. No. 3,953,566. For example, the stretched PTFE layers may besubstantially thinned during stretching whereas prior art “expansion”processes typically leave the thickness of the expanded materialsomewhat unchanged but generate distinct nodal and fibril microstructurealong with increased porosity and permeability in order to accommodatethe expansion of the layer in plane of the layer.

Referring to FIG. 1, a fine PTFE resin powder is compounded with anextrusion agent such as a liquid lubricant to form a PTFE compound 10. Avariety of different PTFE resins may be used such as the lower extrusionratio, higher molecular weight fine powder coagulated dispersion resins(available from 3M Corporation, Ausimont Corporation, DaikinCorporation, DuPont and ICI Corporation) The PTFE molecules used inthese resins typically have an average molecular weight of from about 20million to about 50 million or more. Optionally, an additive, such aspowdered or liquid color pigment or other resin additive may be added tothe PTFE resin and lubricant to change the properties of the final PTFElayer. For example, a fluorinated copolymer may be added (such asperfluoropropylvinylether-modified PTFE) to improve the bondability ofthe PTFE layer. Additive is typically provided in a mass amount that isless than 2% of the mass of the PTFE resin, but it may be provided inany amount that produces a desired result. Additive may be combined withthe PTFE resin before the lubricant is added so as to ensure homogenousmixing of the additive throughout the PTFE resin.

A variety of different types of extrusion and stretching agents, orlubricants, may be compounded with the PTFE powder resin. Some examplesof lubricants that may be mixed with the PTFE resin include, but are notlimited to, isoparaffin lubricants such as ISOPAR® H, ISOPAR® K andISOPAR® M all of which are manufactured by ExxonMobil Corporation.Additional lubricants include mineral spirits, naphtha, MEK, toluene,alcohols such as isopropyl alcohol, and any other chemical that iscapable of saturating the PTFE resin. In addition, two or morelubricants may be blended together for some lubricant embodiments. Theamount of lubricant added to the PTFE resin may vary depending on thetype of lubricant used as well as the desired properties of a final PTFElayer. Typically, however, the percent mass of lubricant for somecompound embodiments may vary from about 15% to about 25% of thecompound mass; specifically, from about 17% to about 22% of the compoundmass, and more specifically from about 18% to about 20% of the compoundmass.

The PTFE resin and lubricant may be mixed until a substantiallyhomogenous PTFE compound 10 is formed. Compounding of the PTFE resin andlubricant is typically carried out at a temperature below the glasstransition temperature of the PTFE resin which is typically from about55° F. to about 76° F. Compounding of the PTFE resin may be carried outat a temperature below about 50° F., and specifically, at a temperatureof from about 40° F. to about 50° F., so as to reduce shearing of thefine PTFE particles. Once mixed, the PTFE compound maybe stored at atemperature of above approximately 100° F., and typically from about110° F. to about 120° F. for a time period that ensures that thelubricant has absorbed through the PTFE resin particles. The storagetime period typically may be greater than about six hours, and may varydepending on the resin and lubricant used.

Once the compounded PTFE resin and lubricant 10 have been suitablyprepared, the compound 10 may be placed in an extruder, such as the ramextruder 12 shown in FIG. 1. The ram extruder 12 includes a barrel 13and a piston 14 that is configured to slide within a chamber of thebarrel 13 and form a seal against an inner cylindrical surface of thebarrel 13. The compound 10 is placed in the chamber of the extruder 12between the distal end of the piston 14 and an extruder die 16 sealed tothe output end 18 of the extruder 12. The ram extruder 12 may alsoinclude heat elements 20 disposed about the output end 18 of the barrel13 which are configured to uniformly heat the output end 18 of theextruder 12. In some methods, the output end 18 of the extruder isheated before the compounded PTFE resin 10 is loaded into the chamber.An embodiment of a ram extruder 12 may include a Phillips ScientificCorporation vertical three inch hydraulic ram extruder.

Once the PTFE resin compound is loaded, the piston 14 is advancedtowards the output end 18 of the extruder 12, as indicated by arrow 21which increases the chamber pressure and forces the PTFE compound 10 tobe extruded through an orifice 22 of the die 16 to form an extrudate 24.The extrudate 24 may be in the form of a ribbon or tape that is thenwound onto a take up spool 26 as indicated by the arrow adjacent thetake up spool in FIG. 1. The ram extrusion process represents amechanical working of the compound 10, and introduces shear forces andpressure on the compound 10. This working of the compound results in amore cohesive material in the form of an extrudate ribbon or tape 24.

Processing conditions may be chosen to minimize the amount of lubricantthat is evaporated from the PTFE extrudate ribbon 24. For example, thePTFE compound 10 may be extruded at a temperature that is above theglass transition temperature, and typically above 90° F. The PTFEextrudate ribbon 24 is generally fully densified, non-porous andtypically has approximately 100% of its original amount of lubricantupon extrusion from the die 16. The die 16 may also be configured toproduce an extrudate 24 having other configurations, such as a tubularconfiguration. Also, for some methods, the PTFE compound 10 may beprocessed to form a preform billet before it is placed in the extruder12. In addition, a de-ionizing air curtain may optionally be used toreduce static electricity in the area of the extruder 12. In oneexample, the ram extruder 12 has a barrel 13 with a chamber having aninside transverse diameter of about 1 inch to about 6 inches indiameter. Embodiments of the die 16 may have orifices 22 configured toproduce an extrudate ribbon or tape 24 having a width of about 1 inch toabout 24 inches and a thickness of about 0.020 inch to about 0.040 inch,specifically, about 0.025 inch to about 0.035 inch.

After extrusion, the wet PTFE extrudate ribbon 24 may be calendered in afirst direction or machine direction, as indicated by arrow 27, toreduce the thickness of the PTFE extrudate ribbon 24 into a PTFE layer28 as shown in FIG. 2. During the calendering process, the width of thePTFE extrudate ribbon 24 and calendered PTFE layer 28 changes littlewhile the PTFE extrudate ribbon 24 is lengthened in the machinedirection. In one embodiment, the PTFE extrudate ribbon 24 andcalendered PTFE layer 28 may be about 6 inches to about 10 inches inwidth. The calendering process both lengthens and reduces the thicknessof the PTFE ribbon 24 to form PTFE layer 28 which is taken up by spool32. During calendering, the PTFE extrudate ribbon 24 may be calenderedbetween adjustable heated rollers 30 to mechanically compress and reducethe thickness of the PTFE ribbon 24. As such, the calendering processalso encompasses a second mechanical working of the compound 10.Suitable equipment for the calendering process includes a custom 12 inchvertical calendar machine manufactured by IMC Corporation, Birmingham,Ala.

While it may be possible to store the PTFE extrudate ribbon 24 for anextended period of time after extrusion, lubricant in the PTFE extrudateribbon 24 will evaporate from the ribbon 24 during the storage period.As such, it may be desirable in some instances to calender the PTFEextrudate ribbon 24 almost immediately after extrusion so as to bettercontrol the lubricant level in the PTFE extrudate ribbon 24. For someembodiments, the PTFE ribbon 24 will have a lubricant content of about15% to about 25% immediately prior to calendering.

Depending on the calendering speed and roller positioning, the PTFEribbon 24 may be calendered down to produce a PTFE layer 28 of anysuitable thickness. The reduction ratio of an embodiment of thecalendering process, which is a ratio of the thickness of the PTFEextrudate ribbon 24 to the thickness of the calendered PTFE layer 28,may be between about 3:1 to about 75:1, and specifically between about7.5:1 to about 15:1. In one particular embodiment, for a PTFE extrudateribbon 24 having a thickness of about 0.030 inches, calendering mayreduce the thickness to about 0.001 inch to about 0.006 inch,specifically, between about 0.002 inch to about 0.004 inch. In someinstances, the PTFE ribbon 24 may be calendered to a PTFE layer 28 whichhas a thickness that is slightly greater than a final desired thickness,so that the final stretch of the PTFE ribbon 24 causes the final PTFElayer 28 to have its desired thickness.

The calendering temperatures and processing parameters may be chosen sothat the calendered PTFE layer 28 still has a significant amount ofresidual lubricant after the calendering process. For this embodiment,the adjustable rollers 30 may be heated to a temperature between about100° F. and about 200° F., and specifically between about 120° F. andabout 160° F. during the calendering process. After calendering, aresidual amount of lubricant will remain in the PTFE layer 28 which maytypically be between about 10% to about 22% lubricant by weightremaining, specifically about 15% to about 20% lubricant by weight.

Once the PTFE ribbon 24 has been calendered to produce PTFE layer 28,PTFE layer 28 may then be mechanically stretched transversely (alsocalled the cross machine direction), in the longitudinal direction (alsocalled the machine direction), both of these directions or any othersuitable direction or combination of directions, in order to thin thePTFE layer 28, generate a suitable microstructure and mechanically workthe PTFE. It should be noted that although this specification describesa process whereby a PTFE layer is stretched transversely, then stretchedlongitudinally and then densified, the order these steps are performedin may be changed. For example, a PTFE layer may be first stretchedlongitudinally, then stretched transversely. Such a layer may optionallythen be densified, as discussed below. For the transverse stretchingprocess shown in FIGS. 3 and 4, a tentering machine 34 may be used tomechanically stretch the calendered PTFE layer 28 into a stretched PTFElayer 36. One embodiment of a suitable tentering machine 34 includes a60 inch wide by 28 foot long tenter having a T-6 10 horsepower driveunit, manufactured by Gessner Industries, Concord, N.C.

For some embodiments, in order to produce desired thickness, porosity,permeability as well as mechanical properties, process parameters suchas temperature, stretch ratios and material lubricant content of PTFElayer 28, may be controlled before and during the stretching process. Assuch, for some embodiments, a stretching agent or lubricant 40 mayoptionally be applied to the calendered PTFE layer 28 during thestretching process as shown in FIGS. 3 and 4. Applying the stretchingagent 40 to the PTFE layer 28 prior to or during the stretching processof the PTFE layer 28 may be used to control the lubricant content of thestretched PTFE layer 36. This technique may be used to providecharacteristics to the stretched PTFE layer 36 such as thinness, lowporosity and low or substantially no permeability. This methodembodiment also allows for the stretched PTFE layer 36 to have a highdegree of limpness and suppleness to allow mechanical manipulation orstrain of such a PTFE layer without significant recoil or spring backwhich may be particularly useful for some applications. If a highdensity, liquid and gas-impermeable PTFE layer 28 is desired, the PTFElayer 28 may be saturated throughout the thickness of the PTFE layer 28with one or more stretching agents 40 during stretching. If a moreporous PTFE layer 28 is desired, a lesser amount of stretching agent 40will be applied onto the PTFE layer 28. Stretching the PTFE layer 28 maybe carried out for some embodiments at a temperature of about 80° F. toabout 100° F., specifically, about 85° F. to about 95° F.

The stretching agent 40 may be the same lubricant used to form the PTFEcompound 10 or it may be a different lubricant or combination oflubricants. In some embodiments, the stretching agent may be applied insufficient quantities to the PTFE layer 28 to saturate the PTFE layer 28during the stretching process. The stretching agent may be applied by avariety of methods to a surface, such as the upper surface 38, of thePTFE layer 28 during the stretching process. For example, the stretchingagent 40 may be sprayed over the entire layer 28, or only on selectedportions of the PTFE layer 28 by a spray mechanism 42 to the uppersurface 38 of the PTFE layer 28. The stretching agent 40 is applied tothe PTFE layer 28 after the PTFE layer 28 unwinds from spool 32 andpasses under the spray mechanism 42. The stretching agent 40 may beapplied uniformly over one or both sides of the PTFE layer 28, on onlyone side of the PTFE layer 28, or only on selected portions of the PTFElayer 28 at a temperature of typically about 70° F. to about 135° F.,specifically, about 105° F. to about 125° F., and more specifically,about 110° F. to about 120° F.

If a PTFE layer having low or substantially no fluid permeability isdesired, the PTFE layer 28 will be stretched in one or more directionswhile fully saturated until the desired thickness is achieved. It shouldbe noted that as the PTFE layer 28 is stretched, the capacity of theresulting stretched PTFE layer 36 to absorb stretching agent 40increases. As such, if it is desirable to maintain a saturated status ofthe PTFE layer 28 and stretched PTFE layer 36, it may be necessary toadd stretching agent multiple times or over a large area in order tomaintain that saturated state of the PTFE layer 36 and the effect of thestretching agent 40 temperature (about 110° F. to about 120° F.) for aperiod of time. As such, stretching agent 40 may be added prior to theinitiation of the stretching process or at any time during thestretching process. A method whereby stretching agent 40 is applied tothe PTFE layer during the stretching process may allow for the formationof discernable node and fibril microstructure creation during thestretching process prior to application of the stretching agent 40 tothe PTFE layer; however, thinning of the PTFE layer will still takeplace once the stretching agent 40 has been applied and stretchingcontinues.

FIG. 4 illustrates the stretching agent 40 being applied to uppersurface 38 of the PTFE layer 28 by spray mechanism 42 as the PTFE layer28 is being stretched transversely. For saturated stretchingembodiments, it may be necessary to apply sufficient stretching agent soas to pool or puddle the stretching agent on the upper surface 38 of thePTFE layer 28. In such a case, the pooled or puddled stretching agentmay be spread over the upper surface 38 of the PTFE layer 28 by askimming member 44 that has a smooth contact edge 46 adjacent the uppersurface 38 of the PTFE layer 28. The skimming member 44 is disposedadjacent the spray mechanism 42 displaced from the spray mechanism inthe machine direction of the PTFE layer 28 such that the stretchingagent 40 applied by the spray mechanism 42 runs into the skimming member44 and is spread by the motion of the stretching agent 40 and PTFE layer28 relative to the skimming member 44. The skimming member 44 may be incontact with the upper surface 38 of the PTFE layer 28 or may also bedisposed above the upper surface 38, depending on the desiredconfiguration of the set up, the type of stretching agent being used aswell as other factors. Multiple skimming members may be used with someor all of the skimming members having a smooth contact edge oralternatively a grooved/patterned contact edge.

Embodiments of methods discussed herein may be useful to reduce athickness of the PTFE layer 28 to a stretched PTFE layer 36 of anythickness down to about 0.00005 inch, but typically from about 0.00005inch and 0.005 inch. Typical transverse stretch ratios may be from about3:1 to about 20:1. In one embodiment, a calendered PTFE layer 28 havinga width of about 3 inches to about 6 inches, may be transverselystretched, as shown in FIGS. 3 and 4, into a stretched PTFE layer 36having a width of about 20 inches to about 60 inches. This represents astretch ratio of about 3:1 to about 12:1. In another embodiment, acalendered PTFE layer 28 having a width of about 3.5 inches to about 4.5inches, may be transversely stretched, as shown in FIGS. 3 and 4, into astretched PTFE layer 36 having a width of about 20 inches to about 60inches. This represents a stretch ratio of about 7.8:1 to about 13:1.

As discussed above, the thickness, porosity, average pore size and fluidpermeability of the PTFE layers 36 may be affected by the amount andtemperature of stretching agent 40 applied to the layer 36 prior to orduring stretching. In addition, the temperature of the PTFE layer, thetype of stretching agent that is applied to the PTFE layer, and thestretch rate may also affect the thickness, porosity, average pore sizeand fluid permeability of the PTFE layer 36. By adjusting theseparameters, these characteristics may be optimized in order to produce aPTFE layer that is suited to a particular application. For example, ifthe PTFE layer 36 is used as a moisture barrier for clothing, theparameters may be adjusted to produce an average pore size of less thanabout 6.0 microns. Alternatively, if the PTFE layer 36 is used in anendovascular graft that benefits from tissue in-growth, the average poresize is adjusted to be greater than 6.0 microns. In other embodiments,where the PTFE layer 36 is a barrier layer for use in an endovasculargraft, the pore size may be smaller, such as between about 0.01 micronsand about 5.0 microns. In addition, embodiments of the stretched PTFElayer 36 are fusible and deformable and may easily be fused with otherPTFE layers having different properties. At any point after the PTFElayer 28 is stretched, the stretched PTFE layer 36 may be sintered toamorphously lock the microstructure of the PTFE layer 36. Sintering maybe performed to combine the stretched PTFE layer 36 with other layers ofPTFE to form multi-layer films, such as those used for endovasculargrafts and the like discussed below.

The stretched PTFE layer optionally may be subjected to a secondstretching process, as shown in FIGS. 3, 4, 5 and 6, wherein thestretched PTFE layer 36 is formed into a twice-stretched PTFE layer 46.Once again, as discussed above, it is important to note that althoughthe method embodiments discussed herein are directed to a firsttransverse stretch and subsequently to a longitudinal or machinedirection stretch, the order of the stretch direction steps may bereversed and other combinations of stretch directions and numbers arealso contemplated. For example, PTFE layer 28 may be stretched twice inthe machine or longitudinal direction without any transverse stretching.PTFE layer 28 may be stretched first in a longitudinal or machinedirection and then in a transverse direction. In addition, a PTFE layer28 may be stretched three or more times. Some or all of the speeds,stretch ratios, temperatures, lubricant parameters and the like asdiscussed herein may be the same or similar to those previouslydescribed, but need not necessarily be so. Moreover, these parameterstypically will not be the same for any of these various stretchingsteps, regardless of the order in which they are undertaken.

This optional second stretching process subjects the PTFE layer 36 toyet another mechanical working. The second stretching process shown inFIGS. 5 and 6 is being carried out in the machine direction; however,the second stretching process may also be carried out in any othersuitable direction, such as transversely. The twice-stretched PTFE layer46 is wound onto spool 48 after undergoing the second stretchingprocess. Additional stretching agent 40 optionally may be applied to asurface of the stretched PTFE layer 36 as the layer 36 is beingstretched a second time. If higher porosity and fluid permeability aredesired, the second stretch may be performed with the stretched layer 36in a dry state without the addition of lubricant during the secondstretch. If the stretched PTFE layer 36 has residual lubricant withoutadditional lubricant added, the second stretching process will generatea microstructure having significant nodes connected by fibrils. Thesecond stretching process may be carried out at a temperature of about85° F. to about 95° F. for some embodiments. The stretch ratio for thesecond stretch may be up to about 20:1, specifically, about 6:1 to about10:1.

If the PTFE layer 28 is stretched in two or more directions, the rate ofstretching in the two directions; e.g., the machine direction and theoff-axis or transverse direction, may have different or the same stretchrates. For example, when the PTFE layer 28 is being stretched in themachine direction (e.g., first direction), the rate of stretching istypically in the range from about two percent to about 100 percent persecond; specifically, from about four percent to about 20 percent persecond, and more specifically about five percent to about ten percentper second. In contrast, when stretching in the cross machine ortransverse direction, the rate of stretching may be in the range fromabout one percent to about 300 percent per second, specifically fromabout ten percent to about 100 percent per second, and more specificallyabout 15 percent to about 25 percent per second.

Stretching in the different directions may be carried out at the sametemperatures or at different temperatures. For example, stretching inthe machine direction is generally carried out at a temperature belowabout 572° F., and for some embodiments, below about 239° F. Incontrast, stretching in the transverse direction is typically carriedout at a temperature above the glass transition temperature, and usuallyfrom about 80° F. to about 100° F. Stretching PTFE layers 28 at lowertemperatures will reduce stretching agent 40 evaporation and retain thestretching agent 40 in the PTFE layer 28 for a longer period of timeduring processing.

Either the stretched PTFE layer 36 or the twice-stretched PTFE layer 46optionally may be calendered in order to further thin and densify thematerial. The twice-stretched PTFE layer 46 is shown being calendered inFIGS. 7 and 8. In this example, the twice-stretched PTFE layer 46 isunwound from spool 48, passed through calender rollers 50 and 52, formedinto a densified layer 54, then taken up on spool 54. The calendermachine may be the same machine or a different machine as that indicatedin FIG. 2 and discussed above. This final calendering or densificationof PTFE layer 46 generally produces a highly densified PTFE layer 54that has no discernable microstructure features, such as pores, and haslow or substantially no fluid permeability. The methods of compressingand stretching PTFE layers may both be used to control thinning of thePTFE layer and the microstructure that results from the thinningprocess. The densified PTFE layer 54 may also lack the suppleness andlimpness mechanical properties of the stretched PTFE layers 36 and 46discussed above. The rollers 50 and 52 may be adjusted to have anysuitable separation to produce a PTFE layer 54 having a thickness ofabout 0.00005 inch to about 0.005 inch. The rollers 50 and 52 may alsobe heated during the calendering process, with typical temperaturesbeing from about 90° F. to about 250° F.; specifically, from about 120°F. to about 160° F.; more specifically, from about 130° F. to about 150°F.

The following example describes specific methods of manufacturing of thestretched PTFE layers 36. In this embodiment, 1000 grams of resin arecompounded with an isoparaffin based lubricant; specifically, ISOPAR® M,in a mass ratio of lubricant-to-PTFE compound from about 15% to about25%. Compounding of the PTFE resin and lubricant is carried out at atemperature below 50° F., which is well below the glass transitiontemperature of the PTFE resin of between about 57° F. to about 75° F.

The PTFE compound 10 may be formed into a billet and stored at atemperature of about 105° F. to about 125° F. for six or more hours toensure that the lubricant substantially has penetrated and absorbedthrough the resin. Thereafter, the PTFE compound 10 is placed in anextruder 12, as shown in FIG. 1. The PTFE compound 10 may then be pasteextruded from the orifice 22 of the die 16 of the extruder 12 at atemperature above the resin glass transition temperature. In oneembodiment, the paste is extruded at a temperature from about 80° F. to120° F. A reduction ratio, e.g., a ratio of a cross sectional area ofthe PTFE compound 10 before extrusion to the cross section area of thePTFE extrudate 24 after extrusion, may be from about 10:1 to about400:1, and specifically may be from about 80:1 to about 120:1. Theextruder 12 may be a horizontal extruder or a vertical extruder. Theorifice 22 of the extrusion die 16 determines the final cross sectionalconfiguration of the extruded PTFE ribbon 24. The orifice 22 shape orconfiguration of the extrusion die 16 may be tubular, square,rectangular or any other suitable profile. It may be desirable topreform the PTFE compound (resin and lubricant) into a billet.

The PTFE extrudate ribbon 24 is then calendered, as shown in FIG. 2, ata temperature from about 100° F. to about 160° F. to reduce a thicknessof the PTFE ribbon 24 and form a PTFE layer or film 28. The temperatureat calendering may be controlled by controlling the temperature of therollers 30 of the calender machine. The PTFE layer may be calendereddown to a thickness from about 0.001 inch to about 0.006 inch, andspecifically, down to a thickness of about 0.002 inch to about 0.003inch. At the end of the calendering, the calendered PTFE layer 28 mayhave a lubricant content of about 10% by weight to about 20% by weight.

Referring again to FIGS. 3 and 4, after calendering, one side or bothsides of the calendered PTFE layer 28 are sprayed with anisoparaffin-based stretching agent 40 at a prescribed temperature sothat the PTFE film or layer 28 is flooded and fully saturated throughthe thickness of the PTFE layer 28. The saturated, calendered PTFE layermay then be stretched in a direction that is substantially orthogonal tothe calendering direction by a tentering machine 34 to reduce athickness of the PTFE layer 28 and form a stretched PTFE layer 36. Thestretched PTFE layer 36 may have a thickness of about 0.00005 inch toabout 0.005 inch; specifically, the stretched PTFE layer 36 may have athickness of about 0.0002 inch to about 0.002 inch. The PTFE layer 28typically is tentered or stretched at an elevated temperature above theglass transition temperature, specifically, from about 80° F. to about100° F., more specifically, about 85° F. to about 95° F.

Wet tentering with the stretching agent 40 allows the PTFE layer 28 tobe thinned without creating substantial porosity and fluid permeabilityin the stretched PTFE layer 36. While the stretched PTFE layer 36 willhave a porosity, its porosity and pore size typically will not be largeenough to be permeable to liquids, and often will be small enough tohave substantially no fluid permeability. In addition, the stretchedPTFE layer embodiment 36 does not have the conventional node and fibrilmicrostructure but instead has a closed cell microstructure in whichboundaries of adjacent nodes are directly connected with each other. Thefluid-impermeable stretched PTFE film or layer 36 typically may have adensity from about 0.5 g/cm³ to about 1.5 g/cm³, but it may have alarger or smaller density for some embodiments. In addition, with regardto all of the methods of processing layers of PTFE discussed above, anyof the PTFE layers produced by these methods may also be sintered at anypoint in the above processes in order to substantially fix themicrostructure of the PTFE layer. A typical sintering process may be toexpose the PTFE layer to a temperature of about 350° C. to about 380° C.for several minutes; specifically, about 2 minutes to about 5 minutes.

In another aspect of the methods and PTFE layers discussed herein, thePTFE layer 28 may be selectively lubricated in a predeterminedhorizontal or lateral spatial pattern with a stretching agent 40. Thepredetermined horizontal spatial pattern may be formed from variouslateral zones or sections which may each have varying levels ofstretching agent 40 content or stretching agent 40 content gradientswithin and/or between lateral zones. Lateral zones of a PTFE layer canextend in any direction across a layer of PTFE, including transversely,longitudinally or any direction in between these directions. Lateralzones of a layer of PTFE are distinguishable from a thickness gradientof stretching agent 40 content whereby the content of stretching agent40 varies stepwise or continuously through the thickness of a layer ofPTFE. Selective application of the stretching agent 40 by spraymechanism 42 to a surface of a layer of PTFE may be carried out usingthe methods described herein or using other conventional methods. Thelevels of stretching agent 40 contained within the various lateral zonesof the PTFE layer 28 may vary from about 0 percent stretching agentcontent by weight to a level of substantial saturation of stretchingagent 40.

PTFE layer 28 having a predetermined pattern of stretching agent 40 maybe stretched in at least one direction such that the lateral zones ofthe stretched PTFE layer 36 that contained more stretching agent 40during stretching will have a lower permeability (e.g., substantiallyimpermeable), while the lateral zones of the stretched PTFE layer 36that contained less stretching agent 40 during stretching will have ahigher permeability. Typically, the lateral zones of the stretched layer36 that contained more stretching agent 40 may also have a reducedthickness relative to the lateral zones that contained less stretchingagent 40 during the stretching process. In some embodiments, it may bedesirable to have lateral zones or regions of the PTFE layer 28 that aresubstantially saturated with stretching agent 40 adjacent lateral zonesor regions of PTFE layer 28 that have a low enough stretching agentcontent to allow expansion of the PTFE layer so as to produce asubstantial node and fibril microstructure with relatively high fluidpermeability. Stretching may be carried out in the machine direction, ina direction that is substantially transverse or orthogonal to themachine direction, or any direction in between. Alternatively, it may bepossible to stretch the PTFE layer 28 radially about a point.

Referring to FIGS. 8A-8D, PTFE layer 28 may have stretching agent 40applied in a predetermined pattern to the PTFE layer 28, such as theexemplary checker board pattern shown in FIG. 8C (which shows anenlarged view of a portion of the PTFE layer 28). This checker boardpattern includes rectangular lateral zones 60 which are substantiallysaturated with stretching agent 40 throughout the thickness of thelateral zones 60 and which are visible on the surface of the layer ofPTFE. The checker board pattern also includes rectangular lateral zones62 which have a significantly lower stretching agent 40 contentthroughout the thickness of the zones 62. Upon stretching, lateral zones60 and 62 transform into lateral zones 64 and 66, respectively, ofstretched PTFE layer 36, as shown in FIG. 8D. Lateral zones 64 and 66are elongated in the transverse direction relative to the length ofzones 60 and 62 because of expansion of PTFE layer 28 in the transversedirection during the tentering process shown in FIGS. 8A and 8B. Lateralzones 60-66 represent areas on the PTFE layer 28 that extend across thesurface of the PTFE layer 28 in any direction or in any shape orconfiguration. The checker board pattern of FIG. 8C is provided forexemplary purposes only. In general, the stretching agent content levelmay be substantially constant throughout a thickness of the PTFE layer28 at any point on the PTFE layer 28 or within a particular lateralzone; however, a stretching agent content level gradient may also bepresent across the thickness of the PTFE layer 28 if desired. Inaddition, while the lateral zones 60 and 62 are described and shown inFIGS. 8C and 8D as being defined by substantially discrete stretchingagent content levels, other embodiments of lateral zones could includeareas of the PTFE layer 28 which include a stretching agent contentlevel gradient in any desired direction or pattern.

FIGS. 8E-8L illustrate alternative embodiments of the effect of applyingpredetermined patterns and amounts of stretching agent 40 on a layer ofPTFE before and after a stretching process, wherein lateral zones 60 aresubstantially saturated with stretching agent 40 and lateral zones 62has a relatively low level of (or substantially no) stretching agent 40.After stretching, lateral zones 60 and 62 transform into lateral zones64 and 66, respectively. In other alternative embodiments, FIGS. 8E and8F show circles of relatively high or substantially saturated stretchingagent 40 content and the elliptical shape the circles may assume afterstretching. FIGS. 8G and 8H show, in another example, a pattern in whichthe rectangular cells of relatively high or substantially saturatedstretching agent 40 content become square in shape after stretching.FIGS. 8I and 8J show elliptical patterns of relatively high orsubstantially saturated stretching agent 40 content that become circularafter stretching. Finally, FIGS. 8K and 8L illustrate a bull's eyepattern that is stretched into an elliptical shape during stretching.

As discussed above, the stretching agent 40 content of lateral zones 62may be chosen such that standard expansion takes place for the PTFEmaterial within lateral zones 62 upon stretching. Standard expansion ofPTFE may produce expanded PTFE (ePTFE) within lateral zones 64 afterstretching, which typically has a substantial node and fibrilmicrostructure that is discernable when viewed in by SEM. Lateral zones60 may be substantially saturated with stretching agent 40 such thatexpansion of the PTFE layer 28 within lateral zones 60 produces PTFEmaterial of lateral zones 64 which is thinner and less permeable thanthe material of lateral zones 66. For some embodiments, the PTFEmaterial of lateral zones 64 may be substantially impermeable and mayhave a closed cell microstructure. The closed cell microstructure mayhave a plurality of interconnected nodes but is substantially free offibrils between the nodes (when viewed at a SEM magnification of 20,000such as shown in FIG. 9). Put another way, the material of lateral zones64, which may be the same as or similar to the material shown in FIG. 9,shows no discernable node and fibril microstructure when viewed by SEMat a magnification of 20,000. In addition, the stretching agent contentof lateral zones 60 and 62 may be chosen such that lateral zones 64 and66 of the stretched PTFE layer 36 may vary with respect to density,thickness, and/or porosity.

The stretching agent 40 may be applied in any preselected lateralspatial pattern and in any desired amount or concentration level withinthe lateral zones of that pattern. For some embodiments, the stretchingagent 40 may be selectively applied to the PTFE layer 28 by a spraymechanism 42 that may be controlled through computer or manual control.The spray mechanism 42 shown in FIGS. 8A and 8B may be configured to becontrollable to a substantial degree of spatial resolution so that finepreselected patterns of stretching agent 40 may be applied to the PTFElayer 28. In some embodiments, the spray mechanism 42 may include aninkjet head, such as is commonly used on an inkjet printer device. Otherembodiments of applying the stretching agent 40 may include, but are notlimited to, a contact roller which may be smooth, textured or grooved.Droplets or stream application may be used with an optional skimmingmember or blade that may also be smooth, textured or grooved. A squeegeethat is smooth, textured or grooved may also be used to spreadstretching agent delivered by droplets or stream spray. Also, a spongethat is smooth, textured or grooved may be used as well as a rotatingdrum having a pattern disposed thereon. Silk screen type of methods andthe like may also be used to apply the stretching agent 40.

For other embodiments of methods of producing PTFE layers having lateralzones of varied fluid permeability as well as other characteristics,selective removal or reduction of stretching agent 40 content from thePTFE layer 28 in a predetermined pattern may be used as opposed toselective addition of stretching agent 40. In such a method, a PTFElayer 28 could be produced having a high level of stretching agent 40content, up to a saturated level, with subsequent removal of some of thestretching agent by the selective application of heat or other energy ina predetermined lateral spatial pattern. The selective application ofheat selectively evaporates or boils the stretching agent from the PTFElayer 28. In such an embodiment, an array of LED lasers 60, or the like,could be disposed adjacent the PTFE layer 28, as shown in FIGS. 3 and 4.The LED laser array 60 could be controlled manually, by a computer orany other suitable means so as to apply laser energy to the PTFE layer28 as it passes by the laser array 60. As such, stretching agent 40 isuniformly applied to the PTFE layer 28 by the spray mechanism 42 andoptional skimming member 44 as shown in FIGS. 3 and 4 so as to produce aPTFE layer 28 having a substantially uniform stretching agent 40 contentlevel. Then, as the PTFE layer 28 passes adjacent the LED laser array60, the individual LED lasers of the array are selectively activated toas to produce a pattern of lateral zones, such as the lateral zonesshown in FIGS. 8C-8L. In addition to laser energy, any other suitableform of energy that can be spatially controlled could also be used. Forexample, radiofrequency energy, ultrasound energy and the like couldalso be used for selective removal or reduction of stretching agent 40from the PTFE layer 28. In addition, air jets or nozzles dispensing airor other gases at a specified temperature, pressure and direction may beused to selectively remove the stretching agent 40 from the PTFE layerby spraying the gas from the air jet onto the PTFE layer to either blowthe stretching agent from the PTFE layer or through the PTFE layer.Also, the gas expelled from such air jets could be heated to facilitateevaporation of the stretching agent from the point of impact of thecompressed or high velocity gas from the air jets.

As discussed above, the PTFE layers 36 may or may not include adiscernable node and fibril microstructure. If the stretched PTFE layers36 or lateral zones of the stretched PTFE layers include a discernablenode and fibril microstructure, the stretched PTFE layers 36 or lateralzones thereof may have a uniaxial fibril orientation, a biaxial fibrilorientation, or a multi-axial fibril orientation. The stretched PTFElayers 36 or twice-stretched PTFE layers 46 within a multi-layer PTFEfilm may be positioned in any configuration, such that the fibrils inone PTFE layer (if any) are parallel, perpendicular, or at other anglesrelative to the fibrils of an adjacent PTFE layer. The stretched PTFElayers with lateral zones can be used on any of the stent graftembodiments discussed below. In some embodiments, it may be desirable touse a stretched PTFE layer with impermeable lateral zones disposed aboutor bordering inflatable channels and permeable lateral zones in otherareas of the stent graft.

The various methods discussed above may be used to produce PTFE layershaving a variety of desirable properties. The scanning electronmicroscope (SEM) images shown in FIGS. 9 to 13 illustrate differentmagnifications of a microstructure of a PTFE film or layer 110 made inaccordance with embodiments of the present invention. PTFE layer 110 hasa generally closed cell microstructure 112 that is substantially free ofthe conventional node and fibril microstructure commonly seen inexpanded PTFE layers. Embodiments of the PTFE film 110 may have lowfluid-permeability, or no or substantially no fluid-permeability. One ormore of PTFE layer 110 may be used as a barrier layer to prevent a fluidsuch as a liquid or gas from permeating or escaping therethrough.

At a magnification of 20,000, as seen in FIG. 9, the microstructure ofthe stretched PTFE layer 110 resembles a pocked-like structure thatcomprises interconnected high density regions 114 and pockets or pores116 between some of the high density regions 114. The PTFE film 110 maybe considered to have a closed cell network structure withinterconnected strands connecting high density regions 114 in which ahigh density region grain boundary is directly connected to a grainboundary of an adjacent high density region. Unlike conventionalexpanded PTFE which typically has a substantial node and fibrilmicrostructure that is discernable when viewed at a SEM magnification of20,000, PTFE layer 110 lacks the distinct, parallel fibrils thatinterconnect adjacent nodes of ePTFE and has no discernable node andfibril microstructure when viewed at a SEM magnification of 20,000, asshown in FIG. 9. The closed cell microstructure of the PTFE layer 110provides a layer having low or substantially no fluid permeability thatmay be used as “a barrier layer” to prevent liquid from passing from oneside of the PTFE layer to the opposite side.

Though PTFE film or layer 110 is configured to have low or substantiallyno fluid permeability, PTFE layer 110 nonetheless has a porosity. ThePTFE layer 110 typically has an average porosity from about 20% to about80%, and specifically from about 30% and about 70%. In one embodiment, aPTFE film 110 has a porosity of about 30% to about 40%. In anotherembodiment, a PTFE layer 110 has a porosity of about 60% to about 70%.Porosity as described in these figures is meant to indicate the volumeof solid PTFE material as a percentage of the total volume of the PTFEfilm 110. An average pore size in the PTFE layer 110 is may be less thanabout 20 microns, and specifically less than about 0.5 micron. In oneembodiment, a PTFE layer 110 has an average pore size of from about 0.01micron to about 0.5 micron. As can be appreciated, if tissue ingrowth isdesired, the PTFE film 110 may have an average pore size of greater thanabout 6.0 microns. As described below, depending on the desiredproperties of the resultant PTFE layer 110, embodiments of methods maybe modified so as to vary the average porosity and average pore size ofthe PTFE film 110 in a continuum from 10 microns to 50 microns down tosubstantially less than about 0.1 micron.

PTFE layer 110 may have a density from about 0.5 g/cm³ to about 1.5g/cm³, and specifically from about 0.6 g/cm³ to about 1.5 g/cm³. Whilethe density of the PTFE film 110 is typically less than a density for afully densified PTFE layer (e.g., 2.1 g/cm³), if desired, the density ofthe PTFE layer 110 may be densified to a higher density level so thatthe density of the PTFE layer 110 is comparable to a fully densifiedPTFE layer. FIGS. 9 to 13 illustrate a PTFE film 110 having a closedmicro structural network and that is substantially impermeable to liquidand gas; other embodiments of PTFE layers may be manufactured using themethods discussed herein to have other suitable permeability values andpore sizes.

PTFE film 110 may have an average thickness that is less than about0.005 inch, specifically from about 0.00005 inch to about 0.005 inch,and more specifically from about 0.0001 inch to about 0.002 inch. Whileembodiments of methods discussed herein are directed to manufacturingPTFE layers, it should be appreciated that the methods discussed mayalso be useful in the manufacture of other fluoropolymer-based filmshaving substantial, low or substantially no fluid permeability. As such,the methods discussed herein are not limited to the processing of PTFEmaterials. For example, the processing of other fluoropolymerresin-based materials, such as copolymers of tetrafluororethylene andother monomers, is also contemplated.

The PTFE layer and PTFE films may be used in a variety of ways. Forexample, the PTFE layers and PTFE films of the present invention may beused for prosthetic devices such as a vascular graft, breast implantsand the like. Other applications include tubing, protective clothing,insulation, sports equipment, filters, membranes, fuel cells, ionicexchange barriers, gaskets as well as others. For some of theseapplications, it may be desirable to include PTFE layers that havevariable characteristics with respect to lateral zones of the PTFElayers, which may be produced by the methods discussed above.Specifically, some applications may require PTFE layers that have a highpermeability in one lateral zone and low or substantially nopermeability in an adjacent lateral zone. PTFE films having at least twoPTFE layers combined may overlap the predetermined patterns of thelateral zones of the PTFE layers to achieve a more complex patterns ofvaried characteristics. As such, any of the embodiments discussed belowmay incorporate PTFE layers, stretched PTFE layers or PTFE films thathave lateral zones of varied characteristics as discussed above.

Referring now to FIG. 14, PTFE layer 110 may be combined with, bondedto, or otherwise coupled, affixed or attached, partially or completely,to at least one additional layer 118 to form a composite film 120.Depending on the use of composite film 120, layer 118 may be chosen tohave properties that combine with the properties of layer 110 to givethe desired properties in composite film 120. The additional layer 118may include a porous PTFE layer, a substantially non-porous PTFE layer,a gas- or liquid-permeable PTFE layer, a gas- or liquid-impermeablelayer, an ePTFE layer, a non-expanded PTFE layer, a fluoropolymer layer,a non-fluoropolymer layer, or any combination thereof. In oneembodiment, layer 118 is a porous, fluid-permeable, expanded PTFE layerhaving a conventional node and fibril microstructure. If desired, one ormore reinforcing layers (not shown) optionally may be coupled to thecomposite PTFE film 120. The reinforcing layer may be disposed betweenlayers 110 or 118, or the reinforcing layer(s) may be coupled to anexposed surface of PTFE layer 110, PTFE layer 118, or both. PTFE layer110 and layer 118 may be combined, bonded to, or otherwise coupled,affixed or attached, partially or completely, to one another using anysuitable method known in the art. For example, an adhesive may be usedto selectively bond at least a portion of layers 110 and 118 to eachother. Alternatively, heat fusion, pressure bonding, sintering, and thelike may be used to bond at least a portion of layers 110 and 118 toeach other.

FIGS. 15 and 16 are transverse cross-sectional views of two compositetubular structures 130 and 140, respectively. Tubular structures 130 and140 may be a portion or section of an endovascular graft or the like. Asshown in FIG. 15, tubular structure 130 includes an inner tubular body132 that comprises an inner surface 134 and an outer surface 136.Tubular body 132 may comprise one or more layers of fluid-permeablePTFE. Such a fluid-permeable layer of PTFE may have a Gurley measurementof less than about 10 Gurley seconds. Tubular structure 130 furthercomprises an outer tubular body 138 that comprises an inner surface 137and an outer surface 139. Inner surface 137 of outer tubular body 138 iscoupled to the outer surface 136 of the inner tubular body 132. Tubularbody 138 may comprise one or more PTFE layers having lowfluid-permeability or substantially no fluid-permeability. In thisconfiguration, inner surface 134 of the tubular body 132 defines aninner lumen 135 of tubular structure 130 and the outer surface 139 ofthe tubular body 138 defines an outer surface 139 of the tubularstructure 130. Tubular body 138 may be combined, bonded to, or otherwisecoupled, affixed or attached, partially or completely, to the tubularbody 132 through any suitable method known in the art. For example, anadhesive may be used to selectively bond at least a portion of tubularbody 138 and tubular body 132 to each other. Alternatively, heat fusion,pressure bonding, sintering, and the like, or any combination thereof,may be used to bond at least a portion of tubular body 138 and tubularbody 132 to each other.

As shown in FIG. 16, tubular structure 140 includes an inner tubularbody 142 that comprises an inner surface 144 and an outer surface 146.Tubular body 142 may comprise one or more layers of PTFE having low orsubstantially no fluid permeability. Tubular structure 140 furthercomprises an outer tubular body 148 that comprises an inner surface 147and an outer surface 149. Inner surface 147 of outer tubular body 148 iscoupled to the outer surface 146 of the inner tubular body 142. Outertubular body 148 may comprise one or more layers of fluid-permeablePTFE. Embodiments of fluid-permeable layers of PTFE may have a Gurleymeasurement of less than about 10 Gurley seconds. In this configuration,inner surface 144 of the inner tubular body 142 defines an inner lumen145 of tubular structure 140 and the outer surface 149 of the outertubular body 148 defines an outer surface 149 of the tubular structure140. Tubular body 148 maybe combined, bonded to, or otherwise coupled,affixed or attached, partially or completely, to the tubular body 142through any suitable method known in the art. For example, an adhesivemay be used to selectively bond at least a portion of tubular body 148and tubular body 132 to each other. Alternatively, heat fusion, pressurebonding, sintering, and the like, or any combination thereof, may beused to bond at least a portion of tubular body 148 and tubular body 142to each other.

Tubular structures 130 or 140 may define an inner diameter ID which isthe diameter of the inner surface, which may define the area of flowthrough tubular structure 130 or 140. An outer diameter OD, which is thediameter of the outer surface 139 or 149 of the outer tubular layer 138or 148. The inner diameter ID and outer diameter OD may be any desireddiameter. For use in an endovascular graft, the inner diameter ID but istypically from about 10 mm to about 40 mm and the outer diameter OD istypically from about 12 mm to about 42 mm. The tubular layers may haveany suitable thickness, however, fluid-impermeable PTFE layers 138 and142 have a thickness from about 0.0005 inch and about 0.01 inch thick,and specifically from about 0.0002 inch to about 0.001 inch. Similarly,fluid-permeable PTFE layers 132 or 148 may also be any thicknessdesired, but typically have a thickness from about 0.0001 inch and about0.01 inch, and specifically from about 0.0002 inch to about 0.001 inch.As can be appreciated, the thicknesses and diameters of the tubularstructures 130 or 140 will vary depending on the use of the tubularstructures.

Tubular structures 130 or 140 may be formed as tubes throughconventional tubular extrusion processes. Typically, however, tubularstructures 130 or 140 may be formed from PTFE layers 110 or 118, asshown in FIG. 14, that are folded on a shape forming mandrel over eachother so that ends of the layers are overlapped and bonded. As anotheralternative, PTFE layers 110 or 118 may be helically wound about theshape forming mandrel to form the tubular structure. Some exemplarymethods of forming a tubular PTFE structure is described in commonlyowned, copending U.S. patent application Ser. Nos. 10/029,557 (whichpublished as US 20030116260 A1) and entitled “Methods and Apparatus forManufacturing an Endovascular Graft Section” and 10/029,584 and entitled“Endovascular Graft Joint and Method of Manufacture”, both filed on Dec.20, 2001 to Chobotov et al., and U.S. Pat. No. 6,776,604 to Chobotov etal., the complete disclosures of which are incorporated herein byreference.

The films and layers discussed herein are not limited to a single porousPTFE layer 118 and a single PTFE layer or film 110 having low orsubstantially no fluid permeability. The composite films 120 and tubularstructures 130 or 140 may include a plurality of porous fluid-permeablePTFE layers (having the same or different node and fibril size andorientation, porosity, pore size, and the like), one or more non-porous,densified PTFE layers, and/or one or more PTFE layers 110 having low orsubstantially no fluid permeability. For example, PTFE layer 110 havinglow or substantially no fluid permeability may be disposed between aninner and outer porous PTFE film or layer. The inner and outer porousPTFE layers may have varying porosities or the same porosities. In suchembodiments, the PTFE layer 110 may have a reduced thickness relative tothe porous PTFE layers. In other embodiments, however, the PTFE layer110 may have the same thickness or larger thickness than the porous PTFElayers. As an alternative embodiment to FIGS. 15 and 16, tubularstructures 130 or 140 may comprise inner and outer tubular bodies thatboth have low or substantially no fluid permeability.

Referring now to FIG. 17, a tubular structure that is in the form of aninflatable endovascular graft 50 is shown. For the purposes of thisapplication, with reference to endovascular graft devices, the term“proximal” describes the end of the graft that will be oriented towardsthe oncoming flow of bodily fluid, typically blood, when the device isdeployed within a body passageway. The term “distal” therefore describesthe graft end opposite the proximal end. Graft 150 has a proximal end151 and a distal end 152 and includes a generally tubular structure orgraft body section 153 comprised of one or more layers of fusiblematerial, including such materials as PTFE and ePTFE. The inner surfaceof the tubular structure defines an inner diameter and acts as a luminalsurface for flow of fluids therethrough. The outer surface of thetubular structure defines an abluminal surface that is adapted to bepositioned adjacent the body lumen wall, within the weakened portion ofthe body lumen, or both. Note that although FIG. 17 shows an inflatableendovascular graft, the layers and films of the present invention may beused in non-inflatable endovascular grafts as well, in addition to othermedical and non-medical applications.

A proximal inflatable cuff 156 may be disposed at or near a proximal end151 of graft body section 153 and a distal inflatable cuff 157 may bedisposed at or near a graft body section distal end 152. Graft bodysection 153 forms a longitudinal lumen that is configured to confine aflow of fluid, such as blood, therethrough. Graft 150 may bemanufactured to have any desired length and internal and externaldiameter but typically ranges in length from about 5 cm to about 30 cm;specifically from about 10 cm to about 30 cm. If desired, a stent 159may be attached at the proximal end 151 and/or the distal end 152 of thegraft 150. Depending on the construction of the cuffs 156 and 157 andgraft body section 153, inflation of cuffs 156 and 157, when notconstrained (such as, e.g., by a vessel or other body lumen), may causethe cuffs 156 and 157 to assume a generally annular or toroidal shapewith a generally semicircular longitudinal cross-section. Inflatablecuffs 156 and 157 may be designed to generally, however, conform to theshape of the vessel within which it is deployed. When fully inflated,cuffs 156 and 157 may have an outside diameter ranging from about 10 mmto about 45 mm; specifically from about 16 mm to about 42 mm.

At least one inflatable channel 158 may be disposed between and in fluidcommunication with proximal inflatable cuff 156 and optional distalinflatable cuff 157. Inflatable channel 158 in the FIG. 17 example has ahelical configuration and provides structural support to graft bodysection 153 when inflated to contain an inflation medium. Inflatablechannel 158 further prevents kinking and twisting of the tubularstructure or graft body section when it is deployed within angled ortortuous anatomies as well as during remodeling of body passageways,such as the aorta and iliac arteries, within which graft 150 may bedeployed. Together with proximal and distal cuffs 156 and 157,inflatable channel 158 forms an inflatable network over the length ofthe body 153. Depending on the desired characteristics of theendovascular graft 150, at least one layer of the graft may be a PTFElayer having low or substantially no fluid permeability such as PTFElayer or film 110. The PTFE layer may be one of the layers that formsthe inflatable channels 158, or the PTFE layer may surround or beunderneath the inflatable channel 158 and cuffs 156 and 157.

In addition, it may be desirable for some embodiments to include a PTFElayer 110 that has varied permeability across lateral zones of the PTFElayer 110 whereby the characteristic of a lateral zone of the PTFE layer110 corresponds to the function of the PTFE layer in the lateral zone.For example, instead of including a single PTFE layer 110 of PTFEmaterial that has substantially no fluid permeability, the layer mayinclude lateral zones of PTFE material that has substantially no fluidpermeability that correspond to the portion of the PTFE layer 110 thatwill be adjacent the inflatable channel 158 and cuffs 156 and 157. Inthis way, the inflatable channel 158 and inflatable cuffs 156 and 157can be made resistant to fluid loss, while the graft body retains itsfluid-permeable character adjacent the inflatable channel 158 andinflatable cuffs 156 and 157. This type of arrangement could be includedin any of the graft embodiments discussed herein.

Graft body 153 may be formed of two or more layers or strips of PTFEthat are selectively fused or otherwise adhered together as describedherein, to form the inflatable cuffs 156 and 157 and inflatable channel158 therebetween. A detailed description of some methods ofmanufacturing a multi-layered graft are described in co-pending andcommonly owned U.S. patent application Ser. Nos. 10/029,557 (whichpublished as US 20030116260 A1); 10/029,584; U.S. patent applicationSer. No. 10/168,053, filed Jun. 14, 2002 and entitled “InflatableIntraluminal Graft” to Murch, and U.S. Pat. No. 6,776,604 to Chobotov etal., the complete disclosures of which are incorporated herein byreference.

FIGS. 18 to 21 illustrate transverse cross sectional views of differentembodiments of inflatable channel 158. As can be appreciated, theembodiments of FIGS. 18 to 21 may also be applicable to the proximal anddistal cuffs 156 and 157. Inflatable channel 158 defines an inflatablespace 162 that is created between an inner layer 164 and outer layer166. If desired an inflation medium 167 may be delivered into the space162 to inflate inflatable space 162. Inflation medium 167 optionally mayinclude a deliverable agent 168 as shown in FIGS. 18 to 21, such as atherapeutic agent 168 that may be configured to be diffused in acontrolled manner or otherwise transmitted through pores (not shown) ininner layer 164, outer layer 166 or both. The embodiments shown in FIGS.18-21 are merely exemplary, as it may be desirable to have preferentialdiffusion of the deliverable agent 168 through layer 164 or layer 166.In addition, both layers 164 and 166 may be configured to allow asignificant amount of diffusion of deliverable agent 168, but with oneof the two layers having a greater permeability to the deliverable agent168 than the other layer. While inner layer 164 and layer 166 are shownas having only a single layer of material, it should be appreciated thateach of layers 164 or 166 may include one or more layers to form acomposite film of fluid-permeable PTFE, PTFE having low fluidpermeability, PTFE having substantially no fluid permeability or anycombination thereof. A more complete description of methods and devicesfor the delivery of a therapeutic agent can be found in copending andcommonly owned U.S. patent application Ser. No. 10/769,532 (whichpublished as US 20050171593 A1), filed Jan. 30, 2004 and entitled“Inflatable Porous Implants and Methods for Drug Delivery” to Whirley etal., the complete disclosure of which is incorporated herein byreference. A description of exemplary inflation medium materials can befound in copending and commonly owned U.S. patent application Ser. No.11/097,467, filed Apr. 1, 2005 and entitled “A Non-Degradable, LowSwelling, Water Soluble, Radiopaque Hydrogel” to Askari et al., thecomplete disclosure of which is incorporated herein by reference.

In the embodiment shown in FIG. 18, outer layer 166 is permeable tofluids so as to allow the therapeutic agent 168, which may be a liquid,to diffuse over time in the direction of arrow 169 through outer layer166. In such embodiments, inner layer 164 typically has a low orsubstantially no fluid permeability, and could therefore be considered a“barrier layer.” Because the inner “barrier” layer 164 has low orsubstantially no fluid permeability and outer layer 166 isfluid-permeable, the therapeutic agent will preferentially diffuse fromspace 162 in the direction of arrow 169. The use of one (or more) porousfluid-permeable outer PTFE layers and an inner layer 164 having low orsubstantially no fluid permeability provides for improved release of atherapeutic agent through liquid-permeable outer layer 166. Varying theporosity or pore size across at least a portion of outer layer 166 mayprovide even more localized delivery of the therapeutic agent 168through outer layer 166.

In an alternative configuration shown in FIG. 19, inner layer 164 may besubstantially fluid-permeable to allow the therapeutic agent 168 toselectively diffuse in the direction of arrow 169 through inner layer164 and into the lumen of the tubular structure (e.g., lumen 135, 145 ofFIGS. 15 and 16). In such embodiments, outer layer 166 typically has noor substantially no fluid-permeability and acts as a “barrier layer.” Assuch, the therapeutic agent will preferentially diffuse from space 162in the direction of arrow 169. The use of porous fluid-permeable PTFElayers and outer layer 166 having low or substantially no fluidpermeability provides for improved release of a therapeutic agent intothe inner lumen through fluid-permeable inner layer 164. Varying thepermeability and/or porosity or pore size across at least a portion ofinner layer 164 may provide even more localized delivery of thetherapeutic agent 168 through layer 164.

As shown in FIG. 20, if it is desired to prevent the inflation medium167 from escaping from inflatable space 162, both the inner layer 164and outer layer 166 may comprise a “barrier” layer having low orsubstantially no fluid permeability. In such embodiments, the inner andouter layers 164 and 166 have low or substantially no fluidpermeability. In such embodiments, inflation material 167 typically willnot contain a therapeutic agent. Referring to FIG. 21, the inflatablechannel may be a substantially tubular channel 170 that is fused orotherwise adhered to layer 164 that defines an inner lumen of the graft.If delivery of a therapeutic agent 168 is desired, tubular channel 170will be liquid-permeable and will allow diffusion of the therapeuticagent 168 through pores in tubular channel 170. If however, it isdesired to prevent the inflation fluid 167 from escaping from inflatablespace 162, then tubular channel 170 will act as a barrier layer and maycomprise at least one layer of PTFE having low or substantially no fluidpermeability.

Referring now to FIGS. 22 and 23, the respective graft embodiments 150and 180 shown include an inflatable channel 158 has portions with acircumferential configuration as opposed to the helical configuration ofthe inflatable channel 158 shown in FIG. 17. The circumferentialconfiguration of portions of the inflatable channel 158 maybeparticularly effective in providing the needed kink resistance forendovascular graft for effectively treating diseased body passagewayssuch as a thoracic aortic aneurysm (TAA), abdominal aortic aneurysm(AAA), in which highly angled and tortuous anatomies are frequentlyfound. In alternative embodiments, other cuff and channel configurationsare possible. Inflatable channel 158 may be configured circumferentiallyas shown in FIGS. 22 and 23.

In addition to the substantially tubular grafts of FIG. 22, bifurcatedendovascular grafts as shown in FIG. 23, are also contemplated. Thebifurcated endovascular graft 180 may be utilized to repair a diseasedlumen at or near a bifurcation within the vessel, such as, for example,in the case of an abdominal aortic aneurysm in which the aneurysm to betreated may extend into the anatomical bifurcation or even into one orboth of the iliac arteries distal to the bifurcation. In the followingdiscussion, the various features of the graft embodiments previouslydiscussed maybe used as necessary in the bifurcated graft 80 embodimentunless specifically mentioned otherwise.

Graft 180 comprises a first bifurcated portion 182, a second bifurcatedportion 184 and main body portion 186. The size and angular orientationof the bifurcated portions 182 and 184 may vary to accommodate graftdelivery system requirements and various clinical demands. The size andangular orientation may vary even between portion 182 and 184. Forinstance, each bifurcated portion or leg is shown in FIG. 23 tooptionally have a different length. First and second bifurcated portions182 and 184 are generally configured to have an outer inflated diameterthat is compatible with the inner diameter of a patient's iliacarteries. First and second bifurcated portions 182 and 184 may also beformed in a curved shape to better accommodate curved and even tortuousanatomies in some applications. Together, main body portion 186 andfirst and second bifurcated portions 182 and 184 form a continuousbifurcated lumen, similar to the inner lumens of FIG. 22, which isconfigured to confine a flow of fluid therethrough. A completedescription of some desirable sizes and spacing of inflatable channelsmay be found in commonly owned, copending U.S. patent application Ser.No. 10/384,103 (which published as US 20040176836 A1), entitled“Kink-Resistant Endovascular Graft” and filed Mar. 6, 2003 to Kari etal., the complete disclosure of which is incorporated herein byreference.

While not shown, it should be appreciated, that instead ofcircumferential channels and longitudinal channels, the bifurcated graft180 may comprise a helical inflatable channel 158, similar to that ofthe graft embodiment shown in FIG. 17 (or other channel geometries toachieve desired results), or a combination of helical andcircumferential channels. A complete description of some embodiments ofendovascular grafts that have helical and cylindrical channelconfigurations may be found in co-pending and commonly owned U.S. patentapplication Ser. No. 10/384,103 (which published as US 2004/0176836 A1).Other endovascular grafts that the liquid-impermeable PTFE film may beused with are described in U.S. Pat. Nos. 6,395,019 to Chobotov,6,132,457 to Chobotov, 6,331,191 to Chobotov, and U.S. patentapplication Ser. Nos. 10/327,711 (which published as US 2003/0125797A1), entitled “Advanced Endovascular Graft” to Chobotov et al. and filedDec. 20, 2002, 10/168,053, the complete disclosures of which areincorporated herein by reference.

As can be appreciated, the inflatable portions of the graft 180optionally may be configured to have varying levels of fluidpermeability and/or porosity, either within or between particular cuffs,channels or cuff/channel segments, so as to provide for controlled drugdelivery, programmed drug delivery or both, into the vessel wall orlumen of the graft via elution of the agent from pores in the layers.For example, any desired portion of the graft 180 may include PTFElayers having low or substantially no fluid permeability. Such aconfiguration would be useful in applications in which the drug deliveryrate and other properties of the graft or stent-graft (e.g. mechanicalproperties) may be selected for the particular clinical needs andindication that is contemplated for that device. In addition, the fluidpermeability and/or porosity may be uniform within a particular cuff orchannel but different between any given channel and/or cuffs. Inaddition to improved drug delivery, the variable porosity of the outersurface of the graft may also be beneficial for promoting tissuein-growth into the graft. It may be possible to make portions of thegraft that are in direct contact with the body lumen to have a higherporosity and/or larger pore size so as to promote tissue in-growth. Inparticular, tissue in-growth may be beneficial adjacent to the proximaland distal ends of the graft.

With regard to the above detailed description, like reference numeralsused therein refer to like elements that may have the same or similardimensions, materials and configurations. While particular forms ofembodiments have been illustrated and described, it will be apparentthat various modifications can be made without departing from the spiritand scope of the embodiments of the invention. Accordingly, it is notintended that the invention be limited by the forgoing detaileddescription.

1. A method of processing PTFE, comprising: providing a layer of PTFE;selectively applying a stretching agent to at least one lateral zone ofthe layer of PTFE in a predetermined pattern; and stretching the layerof PTFE.
 2. The method of claim 1 wherein the layer of PTFE is stretchedwhile the at least one lateral zone is wet with the stretching agent. 3.The method of claim 1 wherein stretching the layer of PTFE comprisesstretching the layer of PTFE by a stretch ratio of about 2:1 to about20:1.
 4. The method of claim 1 wherein the stretching of the layer ofPTFE comprises stretching in a machine direction.
 5. The method of claim1 wherein the stretching of the layer comprises stretching the layer ina direction transverse to the machine direction.
 6. The method of claim1 further comprising calendering the stretched layer of PTFE to compressand densify the PTFE layer.
 7. The method of claim 1 wherein thestretching agent comprises an isoparaffin.
 8. The method of claim 1wherein the stretching agent is selected from the group consisting ofnaphtha, mineral sprits, alcohol, MEK, toluene and alcohol.
 9. Themethod of claim 1 wherein the stretching agent content of the layer ofPTFE prior to selective application of the stretching agent is about 0percent by weight to about 22 percent by weight.
 10. The method of claim1 further comprising stretching the stretched layer of PTFE a secondtime.
 11. A method of processing PTFE, comprising: providing a layer ofPTFE having a stretching agent content level; selectively removingstretching agent from at least one lateral zone of the portion of thelayer of PTFE in a predetermined pattern; and stretching the layer ofPTFE.
 12. The method of claim 11 wherein the layer of PTFE is stretchedwhile at least a portion of the layer of PTFE is wet with stretchingagent.
 13. The method of claim 11 wherein stretching the layer of PTFEcomprises stretching the layer of PTFE by a stretch ratio of about 2:1to about 20:1.
 14. The method of claim 11 wherein the stretching of thelayer of PTFE comprises stretching in a machine direction.
 15. Themethod of claim 11 wherein the stretching of the layer comprisesstretching the layer in a direction transverse to the machine direction.16. The method of claim 11 further comprising calendering the stretchedlayer of PTFE to compress and densify the PTFE layer.
 17. The method ofclaim 11 wherein the stretching agent comprises an isoparaffin.
 18. Themethod of claim 11 wherein the stretching agent is selected from thegroup consisting of naphtha, mineral spirits, alcohol, MEK, toluene andalcohol.
 19. The method of claim 11 further comprising applyingstretching agent to the layer of PTFE prior to selective removal of thestretching agent.
 20. The method of claim 19 wherein the stretchingagent content of the layer of PTFE prior to application of thestretching agent is about 3 percent by weight to about 22 percent byweight.
 21. The method of claim 19 further comprising spreading thestretching agent after application to the layer of PTFE with a skimmingmember disposed adjacent the layer of PTFE.
 22. The method of claim 11further comprising stretching the stretched layer of PTFE a second time.23. A method of processing PTFE, comprising: providing a layer of PTFE;applying a stretching agent to at least one lateral zone of a surface ofthe layer in a predetermined pattern until the lateral zone is saturatedwith the stretching agent; and stretching the layer of PTFE whilelateral zone of the layer of PTFE is saturated with the stretchingagent.
 24. The method of claim 23 further comprising stretching thestretched layer of PTFE a second time.
 25. A PTFE layer comprising alayer made by providing a layer of PTFE; selectively applying astretching agent to at least one lateral zone of the layer of PTFE in apredetermined pattern; and stretching the layer of PTFE.
 26. A PTFElayer comprising a layer made by providing a layer of PTFE having astretching agent content level; selectively removing stretching agentfrom at least one lateral zone of the portion of the layer of PTFE in apredetermined pattern; and stretching the layer of PTFE.
 27. A PTFElayer comprising a layer made by providing a layer of PTFE; applying astretching agent to at least one lateral zone of a surface of the layerin a predetermined pattern until the lateral zone is saturated with thestretching agent; and stretching the layer of PTFE while lateral zone ofthe layer of PTFE is saturated with the stretching agent.
 28. Amulti-layered vascular graft comprising: a first tubular body having anouter surface and an inner surface that defines an inner lumen of thevascular graft; and a second tubular body having an outer surface and aninner surface coupled to the outer surface of the first tubular body,wherein at least one of the first tubular body and the second tubularbody comprises a PTFE layer having a first lateral zone with asubstantially low porosity, a low fluid permeability and no discernablenode and fibril microstructure, and a second lateral zone which isfluid-permeable and has substantial node and fibril microstructure. 29.A tubular structure comprising a layer of PTFE having a first lateralzone which is fluid-permeable and which has substantial node and fibrilmicrostructure and a second lateral zone with a closed cellmicrostructure having high density regions whose grain boundaries aredirectly interconnected to grain boundaries of adjacent high densityregions and having no discernable node and fibril microstructure.
 30. Anendovascular graft comprising a PTFE layer having a first lateral zonewith a liquid-permeable, expanded PTFE layer adjacent a second lateralzone having (a) a closed cell microstructure having high density regionswhose grain boundaries are directly interconnected to grain boundariesof adjacent high density regions and (b) substantially no node andfibril microstructure.
 31. The endovascular graft of claim 30 whereinthe endovascular graft comprises an inflatable endovascular graft havingat least one inflatable channel and wherein the second lateral zonebounds at least a portion of the inflatable channel.
 32. A thin,continuous PTFE layer, comprising: a first lateral zone with asubstantially low porosity, a low fluid permeability, no discernablenode and fibril structure, and a high degree of limpness and supplenessto allow mechanical manipulation or strain of the PTFE layer withoutsignificant recoil or spring back; and a second lateral zone which isfluid-permeable and has a substantial node and fibril microstructure.33. A method of processing a layer of PTFE, comprising: providing alayer of PTFE; stretching the layer of PTFE; and applying a stretchingagent to the PTFE layer during the stretching process.
 34. The method ofclaim 33 wherein the formation of a discernable node and fibrilmicrostructure is created during the stretching process prior toapplication of the stretching agent to the PTFE layer.